[gmx-users] pressure average in 3.3.1

David van der Spoel spoel at xray.bmc.uu.se
Mon Dec 31 10:29:58 CET 2007

Mark Abraham wrote:
> reich at mpikg.mpg.de wrote:
>> Dear Gromacs-users,
>> I have claculated pressures for a trajectory using g_energy. However, the
>> average pressure that is calculated by the program automatically is quite
>> different from the value I get when I average over the data in the output
>> file that was generated by g_energy. So is the output correct or faulty?
> Reporting some of these averages and the time ranges over which you 
> think they have been measured would be helpful.
>> I have checked the mailing list and some users seemed to have the same
>> problem with earlier Gromacs-versions -although I haven´t found any post
>> that reported this for 3.3.1.
> Check out appendix C of the GROMACS manual.

Indeed. The averages that g_energy reports are based on more data than 
what is presented in the .xvg files (which are instantaneous pressures). 
If you set nsteout = 1 you will find that the numbers are identical.

> Mark
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David van der Spoel, Ph.D.
Molec. Biophys. group, Dept. of Cell & Molec. Biol., Uppsala University.
Box 596, 75124 Uppsala, Sweden. Phone:	+46184714205. Fax: +4618511755.
spoel at xray.bmc.uu.se	spoel at gromacs.org   http://folding.bmc.uu.se

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